The invention relates to a turbomolecular pump with a pump stator, a fast rotating pump rotor and a motor for driving the pump rotor.
In a turbomolecular pump, a gas or gas particles are compressed by rotating blades of the pump rotor and the stationary blades of the pump stator to a multiple of the supply pressure to generate a high-vacuum. The gas heating caused by the gas compression and gas friction is mainly dissipated again. via the pump rotor and the pump stator. While the cooling of-the pump stator can be effected by cooling channels carrying a cooling fluid, the active pump rotor cooling is problematic since no cooling fluid can be supplied to the rotating pump rotor. Under unfavorable operational conditions, the pump rotor may therefore overheat. In case of an overheating of the pump rotor beyond a maximally permissible rotor temperature, there is the danger of destroying the pump rotor and, as a consequence, the pump stator. Therefore, the turbomolecular pump always has to be operated below the maximally permissible rotor temperature.
A direct measurement of the rotor temperature is only possible at great efforts because of the difficult signal transmission from the fast rotating pump rotor to the stator. Therefore, the turbomolecular pump comprises a control device restricting the motor output power to a predetermined constant maximum motor output power so that the pump output power and the gas and rotor heating correlating therewith are restricted to a constant maximum value as well.
The permissible maximum motor output power is detected by calculating and/or experimentally by assuming the most unfavorable process conditions for the pump operation, such as a gas with a thermally unfavorable behavior, a bad pump stator cooling, high ambient temperatures etc. The permissible maximum motor output power is selected so that the pump rotor cannot exceed the maximally permissible rotor temperature even under the most unfavorable process conditions. By fixing a constant maximum motor output power, the motor output power is restricted to the predetermined maximum output power even if the process conditions are more favorable than assumed for calculating the maximum motor output power. Thus, the motor output power is restricted to the predetermined maximum motor output power even if the actual rotor temperature has not reached the maximally permissible rotor temperature yet. Since the extreme process conditions underlying the detection of the maximally permissible maximum motor output power only represent a rare exceptional case in practice, the output power of the turbomolecular pump is normally restricted to a value far below an actually thermally permissible value.
Therefore, it is an object of the invention to provide a device and a method by means of which the output power of a turbomolecular pump is increased.